Signal phase control circuits



Nov. 12, 1968 F. w. LESCINSKY 3,411,090

SIGNAL PHASE CONTROL CIRCUITS Filed Dec. 17, 1965 2 Sheets-Sheet 1 FIG.

CARR/ER /3 TRANS/ TION 8 TIM/N6 DETECTOR REco VERY DENS/TY 22 DETECTOR/9- EYE-FINDER PULSES 2 l6 SYMBOL PHASE RECOVERY CCTS 4O SAMPLINGPULSES\ A 3-2 PG 1 4/ 3a'\ CONVERTER DRIVE) REV. COUNTER (DIRECT/ON 29 f32 EYE- F/NbER /-/-//.9/r OCT.

i? APERI'URE PULSES N 3/ REM COUNTER CONVERTER 36 1 APL'RTURE a 27 Ao/REc T/ON J CONTROL p G 24 GENERATOR //v VE/V ToR E W L E S C/NSK Y 5ATTORNEY NOV. 12, 1968 w, gsc sK 3,411,090

SIGNAL PHASE CONTROL CIRCUITS Filed Dec. 17, 1965 Sheets-Sheet 2 FIG. 2

6 LEVEL EVE q ll III I l l|||| I PROD. DENS/ TY OE TRA NS/ 7'/ ON PULSESSH/FTED SYMBOL CLOCK 4 LEAD 36 SH/FTED SAMPLING PULSES 5 5540 37 SH/FTEDAPERTURE PULSES 6 LEAD 33 APERTURE PULSES 7 LEAD 33 SAMPL/NG 505555 FIG. 8 LEAD 3? United States Patent 3,411,090 SIGNAL PHASE CONTROLCIRCUITS Frank W. Lescinsky, Middletown Township, Monmouth County, N.J.,assignor to Bell Telephone Laboratories, 1corporated, New York, N.Y., acorporation of New ork Filed Dec. 17, 1965, Ser. No. 514,583 Claims.(Cl. 325323) This invention relates to automatic phase control circuits.More particularly, the invention relates to circuits for securing apredetermined phase relationship between two electrical signal waveshaving different characteristics.

In certain transmission systems it is necessary to utilize a locallyproduced oscillation wave to control the sampling of another electricalsignal wave. The latter signal wave may, for example, be a datainformation wave or other similar type of wave including successiveinformation-determining signal levels. In such arrangements it isnecessary to achieve an initial phase full-in between the oscillationand signal waves and thereafter to maintain a predetermined phaserelationship between them in spite of dynamic disturbances which canalter the initial phase conditions.

It is usually advantageous to accomplish phase control between twoelectric signal waves by automatic arrangements so that it is notnecessary for an attendant to maintain constant surveillance of thephase relationships. One such automatic phase control system isdisclosed and claimed in the copending application of F. K. Becker andF. W. Lescinsky, Ser. No. 459,589, filed May 28, 1965. The presentinvention is an improvement upon that Becker et al. application and isparticularly useful in fixed transmission circuits that have establishedequalization so that automatic adaptive equalization is not required.The in vention is nevertheless useful in connection with circuits whichutilize such automatic equalization.

In the aforementioned Becker-Lescinsky application circuits are shownfor automatically controlling the phase of timing signals with respectto a multilevel coded data signal by utilizing certain signal amplitudecharacteristics that are relatively stable even though the signal may besubject to substantial distortion prior to reception. The phase controltechnique is based upon the data signal eye pattern characteristics,which pattern is produced when oscilloscope traces of successivesegments of a multilevel data signal are superimposed upon one anotherin a fashion that is known in the art. However, the actual production ofsuch an eye pattern is not required for the present invention or for theinvention in the Becker- Lescinsky application. In that application thedata signal transitions between adjacent information-determinant signallevels are detected and utilized to produce a pulse train which isintegrated by a reversible binary counter for developing an analogsignal to control the phase of a train of timing pulses.

The manner in which the counter direction of operation and drive arecontrolled in the Becker-Lescinsky system causes the timing pulse phaseto be adjusted initially with respect to an amplitude peak in theenvelope of the long time probability distribution of data signaltransitions. Thereafter the timing pulse phase is adjusted with respectto a predetermined aperture time slot wherein the input multilevelsignal has significant information-representing levels. The transitionof phase adjustment between the initial and subsequent control modes isachieved in cooperation with an automatic equalization circuit whichadapts its operation to transmission characteristics of the circuit overwhich the multilevel signals are being received. These two modes ofcontrol operation produce rapid pull-in to the desired phase condition.

'ice

In the Becker-Lescinsky application and in the present application, theaforementioned transition pulses are designated eye-finder pulsesbecause they are used to achieve coarse pull-in of timing signal phaseto the phase of the eye of the data wave. Aperture pulses are generatedto be somewhat longer than the eye time slot and are used in conjunctionwith eye-finder pulses to maintain fine phase lock on the eye in spiteof signal distortion that might shift the position of the aforementioneddistribution envelope characteristic.

It is one object of the present invention to improve electric signalphase control systems.

It is another object to improve automatic arrangements for controllingthe phase relationship between two electric signal waveforms.

Another object of the invention is to achieve rapid phase pull-in fortiming signals without the requirement of an automatic equalizer. I

A further object is to control the phase relationship between twoelectric signal waves so that the exercise of the phase control isrelatively independent of signal distortion.

These and other objects of the invention are realized in an illustrativeembodiment thereof by adjusting the phase of a timing wave in twoseparate branch circuits to a predetermined relationship with respect toa known characteristic of an input data signal wave. After a coarsephase adjustment has been accomplished, circuits which are responsive tothe reduction in the magnitude in phase error angle reduce the rate oferror angle correction in the one of the two branch circuits whichprovides the desired phase adjusted timing signals.

It is one feature of the invention that each control branch circuitincludes a phase adjusting circuit that is operated by a reversiblecounter which in turn is driven by data signal transition pulses derivedfrom the input data signal and which counter is periodically reversed inresponse to a function of the timing wave.

It is another feature that only one of the two branch circuits providescontrolled timing pulses for utilization, and the counter thereof isdriven at a first rate by eyefinder pulses which are in coincidence withaperture pulses, and the same counter is also driven initially at agreater rate by the eye-finder pulses alone.

In accordance with a further feature the counter drive in response toeye-finder pulses alone is inhibited after coarse phase pull-in has beenachieved.

Yet another feature is that the status of the phase relationship betweenthe data and timing waves is detected as a function of the outputs ofthe two phase adjusting branches, one with only phase adjusted timingpulses and the other with phase adjusted aperture pulses.

A more complete understanding of the present invention and the variousfeatures and objects thereof may be obtained from consideration of thefollowing detailed description when considered in connection with theappended claims and the attached drawing in which:

FIG. 1 is a simplified block and line diagram of a phase control systemin accordance with the invention; and

FIGS. 2 through 8 are wave diagrams illustrating the operation of theinvention.

The phase recovery circuits of the invention are illustrated in FIG. 1in only block and line diagram form because the various blocks of thecircuits are either well known in the art or are shown in detail in theaforementioned Becker-Lescinsky application. In either case, the detailsof such blocks comprise no part of the present invention.

The wave diagrams of FIGS. 2 through 8 correspond to the similar diagramin FIGS. 17 through 23 of the men tioned Becker-Lescinsky application.FIG. 2 includes superimposed data symbol traces in a multilevel codeddata wave and illustrates specifically the relevant portions of an eyepattern for a distorted eight-level data signal. However, the inventionis not restricted to operation with signal waves of that number oflevels. The seven eyes in the pattern for such a signal are shown in theleft-hand portion of FIG. 2, and in the central and right-hand portionsof the figure are shown the eight possible alternative signal excursionsthat may be realized in a transition from a single one of the traces atthe left-hand portion of the diagram to a succeeding eye time slot atthe right-hand edge of the figure. Each of the other superimposed tracesof the eye pattern in FIG. 2 may similarly move to its same level or anyone of the seven other informationdeterminant amplitude levels betweensuccessive information-determinant time slots of the data signal, Whichtime slots are represented by the data eye. Superimposed upon the wavediagram of FIG. 2 are seven horizontal lines defining the eightinformation-determinant levels and representing informationdefiningreference levels.

Each time that a data signal excursion in FIG. 2 crosses one of theseven reference levels a transition pulse is generated, as will behereinafter outlined; and a long time probability distribution for suchtransition pulses over a time interval including somewhat more than acomplete data signal symbol interval is shown in FIG. 3. Thedistribution between successive eyes, as indicated in FIG. 3, has anenvelope having a characteristic salient peak approximately midwaybetween successive eyes. This peak occurs at the spectral line 10, andit is not exactly midway between the two eyes because distortion in thedata Signal wave has shifted the time of most frequent intersection ofthe wave with the reference levels to the right in the drawing.

A portion of an additional distribution pattern over a similar datasymbol time interval is indicated in the righthand portion of FIG. 3 forconvenience in considering the remaining wave diagrams, and thisadditional portion of the distribution diagram in FIG. 3 is separatedfrom the characteristic distribution for the complete interval by aninterval 14 of no spectral lines. The latter interval corresponds to thedata signal eye at the right-hand edge of FIG. 2 wherein the data signalwave has some information-representative level and is not crossing anyof the aforementioned seven reference levels. A similar interval 14'corresponds to the eye at the left-hand side of FIG. 2. Thischaracteristic absence of signal transitions at each eye is utilized asa reference for adjusting the phase of a timing signal wave with respectto the data signal wave.

Returning now to FIG. 1, multilevel coded data signals are received froma transmission line, not shown, by an automatic gain control circuit 11.Such signals are modulated on a carrier wave as is known in the art.Equalization for the transmission line is assumed to be provided on theline as is known in the art and is not shown in the drawing. Theautomatic gain control circuit 11 stabilizes the amplitudes of receivedsignals to compensate to a certain extent for amplitude variations thatmay occur in the transmission line. The output of gain control circuit11 is supplied both to a demodulator 12 and to a carrier and timingrecovery circuit 13. The demodulator can be any type known in the art,and the demodulator which is advantageously employed is that shown inthe copending F. K. Becker and L. N. Holzmarr application Ser. No.459,555, filed May 28, 1965. The recovery circuit 13 is advantageouslythat shown in the copending F. K. Becker application Ser. No. 459,659,filed May 28, 1965.

One output from the recovery circuit 13 is coupled to the demodulator 12and includes a carrier frequency signal with any appropriate carrieroffset which may have been injected in the transmission line. A secondoutput from the recovery circuit 13 comprises a timing wave having afrequency which is related to the symbol frequency of the multileveldata signal but which is not necessarily in phase synchronization withthe data signal. This latter timing signal is applied to symbol phaserecovery circuits 16 which are adapted to accordance with the presentinvention to achieve phase synchronization of the timing signal with thedata signal wave.

The demodulator 12 removes the multilivel coded data signal from itscarrier and couples the coded data symbols to a bit rate selectioncircuit 17 such as is shown in the aforementioned Becker-Lescinskyapplication. The selec tion circuit 17 is here considered to includerectifier-slicer circuits which initiate the decoding of the demodulatoroutput for the particular number of coding levels employed in themultilevel data signal. Selection circuit 17 also includes circuits forselecting time constants throughout the receiving circuit of FIG. 1which correspond to the bit rate of binary coded data prior to itscoding in the multilevel form.

The output of bit rate selection circuit 17 is coupled to a decoder 18which produces the ultimate binary coded output signal under the controlof sampling pulses provided on a circuit 19 from the phase recoverycircuits 16. The output of bit rate selection circuit 17 is also appliedto a transition detector circuit 20 which is responsive to such outputfor producing a transition pulse each time the multilevel data signalcrosses one of the information-determinant reference signal levelsherebefore mentioned in connection with FIG. 2. These transition pulsescorrespond to the previously mentioned eyefinder pulses and are appliedto one input of a coincidence gate 21. The second input to the gate 21is also supplied from the transition detector 20 but by way of atransition density detector 22. The latter detector enables the gate 21only when data signal transitions are ocurring at a suflicient rate,i.e., with sufficient density, to provide reliable information about thephase of the multilevel data signal wave. Details of such a detector areincluded in the Becker-Lescinsky application, but the threshold densitylevel about which the detector operates must be determined empiricallyfor any particular system because it depends upon such factors as theextent of equalization, the character of the transmission line, and theerror rate that can be tolerated. Eye-finder pulses in the output ofgate 21 are coupled to the symbol phase recovery circuits 16.

Symbol phase recovery circuits 16 include two branch circuits 24 and 25in which phase adjustments of timing pulses are accomplished. A firstone of these branch circuits is the main phase control branch 24 whichincludes a phase adjusting variable delay circuit 23, an aperture anddirection control generator 26, and a pulse generator 27 for supplyingto the circuit 19 phase adjusted sampling pulses to be used by thedecoder 18. An auxiliary branch circuit 25 including a phase adjustingvariable delay circuit 28 also receives the timing signals from therecovery circuit 13, and its function will be subsequently described.The pulse generator 27 advantageously includes a Schmitt trigger type ofcircuit which receives input pulses from the generator 26 and producescorresponding output pulses of known duration on the circuit 19. Thevariable delay circuit 23 is of the same type shown in theaforementioned Becker-Lescinsky application and comprises at least onetrigger circuit with a voltage controllable triggering threshold. Aplurality of such stages in tandem are advantageously employed. Thevariable delay circuit 28 is of the same type. However, because of thenature of the phase recovery circuit of the present invention, the totalnumber of stages in the delay circuits 23 and 28 need be no more thanthe number of delay stages employed in phase recovery circuits of thetype shown by the Becker-Lescinsky application. The reason for this willbecome apparent as the present phase recovery circuits are furtherdescribed.

Aperture and direction control generator 26 responds to the phaseadjusted timing wave for producing three output signals in much the samemanner that the corresponding signals are produced by similar circuitryin the Becker-Lescinsky application. One of these output signals, on acircuit 33, is a train of recurring aperture pulses in which the pulsesrecur at the symbol rate of the multilevel data signal. Aperture pulsesare shown in FIGS. 6 and 7; and each such aperture pulse has a timeWidth which is somewhat wider than the data eye, e.g., theinformation-representative interval 14 of the data wave wherein thereare no reference level transitions. However, aperture pulse width issubstantially less than the full data symbol period.

The phase relationship between the aperture pulses and the data signalwave is necessarily a direct function of the phase relationship of thetiming wave in the output of the variable delay circuit 23 to the samedata signal wave. The aperture pulses are utilized in an eye-finderinhibit circuit 29 in a fashion which will be subsequently described.The same pulses are also utilized to control the application ofeye-finder pulses to a driving input connection of a reversible binarycounter 30. This control is exercised through a coincidence gate 31which also reeives the eye-finder pulses from gate 21, after inversionin a further gate 32. The aperture pulses appearing in the output of thegenerator 26 are illustrated for two different phase conditions in FIGS.6 and 7, respectively.

A second output from the generator 26 appears on a lead 36 which iscoupled to the reversible counter 30 for controlling the direction ofoperation thereof. This second output is illustrated in FIG. 4 and isdesignated the symbol clock signal. It comprises a cyclic clock wavewith a period equal to the symbol rate of the received data wave andhaving symmetrial positive-going and negativegoing portions. Each signaltransition in the symbol clock wave on circuit 36 reverses the directionof operation of counter 30 so that this counter is reversed at twice thesymbol rate of the data wave. One of these two reversals occursapproximately midway in an aperture pulse, as can be seen by comparingFIGS. 4 and 6, and the other reversal occurs approximately midwaybetween aperture pulses as can be seen from the same comparison.Direction control generator 26 need not include gating circuits forreversing the phase of the symbol clock signal in the manner provided inthe Becker-Lescinsky application. The reason for this is that in thatapplication the phase adjustment was accomplished first with respect totraining pulses occurring approximately in the time slot of the spectralline during a start-up period and subsequently with respect to theaperture pulses and the data eye during normal operation. However, inaccordance with the present invention, both initial coarse phase pullinand subsequent fine phase adjustments are accomplished with respect tothe aperture pulses and the data eye so that it is not necessary tochange the phase of reversals of counter 30 between such two modes ofoperation.

The second output from the generator 26 is also coupled by a circuit 37for application to the pulse generator 27. The pulses in the output ofgenerator 27 are the sampling pulses which are illustrated in FIG. 8 andwhich are applied by circuit 19 to decoder 18.

Counter 30 is driven at either of two rates of operation, as will besubsequently described, to control delay circuit 23. Output signalsrepresenting instantaneous count level conditions are applied to adigital-to-analog converter 38 which responds to such digital signalsfor developing an analog control signal that is utilized for adjustingthe amount of delay in the delay circuit 23. A fast initial drive isprovided for the counter 30 to achieve rapid phase pull-in. This driveis controlled primarily by eye-finder pulses in the output of the gate32 which are coupled through a coincidence gate 39 to a predeterminedintermediate stage of the counter.

The same eye-finder pulses are additionally coupled to the leastsignificant stage of the counter by the gate 31 whenever such pulses arein time coincidence with an aperture pulse on the circuit 33. It will besubsequently shown that when coarse pull-in has been achieved, theeye-finder inhibit circuit 29 disables the gate 39 so that counter 30 isthereafter driven by only those eye-finder pulses which are in timecoincidence with an aperture pulse, and this drive is at a lower ratethan was the aforementioned initial period drive. The number of counterstages included between the least significant stage input and 'the stageat which the output of gate 39 is received must be adapted to the needsof the overall data transmission system as a compromise between pull-inspeed and the hold-in time constant of the phase recovery circuits.Thus, if an excessive number of stages are included between these twoinput connections there would be a long time lag between an indicatedneed for phase adjustment and the actual accomplishment of suchadjustment. However, at the other extreme, if no intermediate stageswere provided, the eye-finder pulses alone and the eyefinder pulses incoincidence with aperture pulses would exercise a wide swinging controland thereby prolong phase pull-in time. For example, the use of fourcounter stages between the inputs from gate 31 and gate 39 would producean advantageous compromise between pull-in speed and hold-in timeconstant for a data transmission system operating in a sixteen levelmode with 2400 multilevel symbols per second.

The auxiliary phase recovery branch circuit 25 is utilized fordetermining when coarse phase pull-in has been completed. The circuitsof the auxiliary adjusting branch are similar to those of the mainbranch in that the delay circuit 28 is controlled by signals from adigitalto-analog converter 38 which represent counting level conditionsof a reversible binary counter 30. The latter counter is driven byeye-finder pulses from the output of gate 21, and its direction ofoperation is controlled by a pulse wave which is a function of theoutput of the delay circuit 28.

A frequency dividing circuit 40 receives the output of the digital delaycircuit 28 and advantageously divides the frequency of such output bytwo for application to control the direction of operation of the counter30'. This direction control wave is essentially the same as the symbolclock wave in FIG. 4. The counter controlled thereby, however, has fewerstages than does the counter 30 because it includes only stagescorresponding to those stages of counter 30 which respond to the outputof gate 39. Thus the counter 30 is advantageously operated atsubstantially the same rate as the counter 30 is operated during theinitial pull-in mode. The output from the divide-by-two circuit 40 isalso coupled through a pulse generator 41, which is similar to the pulsegenerator 27, to another input of the eye-finder inhibit circuit 29'.Thus, pulses in the output of the pulse generator 41 occur at the symbolrate of the data wave and are of short duration so that they appear inmuch the same form as the sampling pulses in either FIG. 5 or FIG. 8.

Inhibit circuit 29 is adapted to indicate the occurrence of successivepulses of the sampling pulse type from generator 41 in time coincidencewith aperture pulses on circuit 33. When, for example, ten successiveones of such pulses from generator 41 have appeared in coincidence withaperture pulses, it is assumed by the system that coarse phase pull-inhas been achieved, and gate 39 is disabled so that counter 30 may bedriven only by eyefinder pulses at its input of least significance.

The inhibit circuit 29 includes a coincidence gate 42 which receives thetwo input signals from the generator 41 and from the circuit 33. Whensuch signals are in time coincidence a monopulser 43 is actuated toproduce a pulse of predetermined fixed amplitude and duration. Theoutput to monopulser 43 is coupled through a lowpass filter 46 to theinput of a slicer circuit 47. Filter 46 has an upper cutoff frequencysomewhat below the symbol rate of the data system and integrates themonopulser output so that the output voltage from the filter increasesrapidly upon the occurrence of a train of successive pulses fromgenerator 41 in coincidence with aperture pulses. This amplitude changein the output of filter 46 actuates slicer 47 to disable the gate 39.However, if during system operation phase lock between the timingsignals and the data signals should be lost for some reason, theaforementioned repetitive coincidences would not occur; and the changein output signal level from filter 46 then causes slicer 47 to enablegate 39 once more so that counter 30 is driven at its high rate toachieve fast pull-in once more for restoring the phase synchronizationrequired for accurate decoding.

Thus, the symbol phase recovery circuits 16 include a main controlbranch path for adjusting the phase of timing signals utilized toproduce sampling pulses, and they also include an auxiliary controlbranch and an inhibit circuit for indicating when a coarse phase pull-incondition has been achieved. These circuits initially cause fast phasepull-in and thereafter reduce the rate of timing signal phase adjustmentto a fine control basis, so that the desired phase relationship may beheld by a strong control with a minimum amount of phase jitter.

Although the present invention has been described in connection with aparticular embodiment thereof, it is to be understood that additionalembodiments and modifications which will be apparent to those skilled inthe art are included within the spirit and scope of the invention.

What is claimed is:

1. In combination first and second circuits including first and secondphase adjusting means, respectively,

means supplying timing signals to said first and second circuits,

means supplying a train of irregularly occurring pulses having over apredetermined time interval a long time distribution probability with anenvelope having a predetermined amplitude characteristic,

first and second controlling means each receiving an output of adifferent one of said first and second adjusting means and alsoreceiving said irregularly occurring pulses, said controlling meansbeing coupled to control said first and second phase adjusting means,respectively, to shift said timing signals toward a predetermined phaserelationship with respect to said distribution characteristic,

means responsive to an output of said first adjusting means generatingrecurring pulses with a period substantially the same as said intervalbut with a pulse width which is much smaller than said interval, and

means responsive to coincidence of said recurring pulses and the timingsignals from said output of said second adjusting means inhibiting theapplication of said irregularly recurring pulses to said firstcontrolling means.

2. The combination in accordance with claim 1 in which said inhibitingmeans comprises a coincidence gate having input connections coupled toreceive an output of said second adjusting means and said recurringpulses,

frequency detection means producing an output signal amplitude change ofpredetermined character as the repetitionrate of output pulses from saidcoincidence gate changes through a predetermined reference rate level,and

means coupling said detection means to inhibit the application of saidirregularly occurring pulses to said first control means whenever saidrepetition rate exceeds said reference level.

3. The combination in accordance with claim 1 in which each of saidcontrolling means comprises a reversible counter driven by saidirregularly occurring pulses, means responsive to the output of saidadjusting means controlled by and corresponding to such controllingmeans reversing said counter approximately midway between each twosuccessive ones of said recurring pulses, and means coupling the outputof said counter to control said corresponding adjusting means. 4. Thecombination in accordance with claim 3 in which said counter outputcoupling means includes a digitalto-analog converter developing ananalog control signal having a magnitude corresponding to the magnitudeof a prevailing count level in said counter, and means applying saidcontrol signal to said adjusting means for controlling the phase of saidtiming signals in accordance with the count level in said counter. 5.The combination in accordance with claim 1 which comprises in addition acoincidence gate having an output thereof coupled to said firstcontrolling means, and means coupling to said gate said recurring pulsesand said irregularly occurring pulses for actuating said firstcontrolling means in response to coincidence of such pulses. 6. Thecombination in accordance with claim 5 in which said inhibiting meanscomprises a coincidence gate having input connections coupled to receivean output of said second adjusting means and said recurring pulses,frequency detection means producing an output signal amplitude change ofpredetermined character as the repetition rate of output pulses fromsaid coincidence gate changes through a predetermined reference ratelevel, and means coupling said detection means to inhibit theapplication of said irregularly occurring pulses to said first controlmeans whenever said repetition rate exceeds said reference level. 7. Thecombination in accordance with claim 5 in which each of said controllingmeans comprises a reversible counter driven by said irregularlyoccurring pulses, means responsive to the output of said adjusting meanscontrolled by such controlling means reversing said counterapproximately midway between each two successive ones of said recurringpulses, and means coupling the output of said counter to control thelast-mentioned adjusting means. 8. The combination in accordance withclaim 7 in which said first and second control means receive saidirregularly occurring pulses for driving said counters thereof atsubstantially the same rate. 9. The combination in accordance with claim8 in which said means coupling said recurring and irregularly occurringpulses in coincidence actuate said counter of said first control meansat a lower rate than the rate of operation thereof in response to saidirregularly occurring pulses. 10. In combination, means receiving amultilevel coded data signal including a train of data symbols occurringat a predeter mined rate, means generating a transition pulse each timesaid signal includes a signal excursion between adjacent ones ofmultiple information-determinant levels thereof, means receiving a trainof timing signals, means responsive to said timing signals generating atrain of regularly recurring pulses, the latter pulses occurring at saidpredetermined symbol rate of said data signal and having a pulse widthwhich is sub- 9 10 stantially less than the period of said signalsymbols, References Cited means integrating said transitionpulses, antiUNITED STATES PATENTS means ad usting the phase of said timing signals,sald 3 206 681 9 9 B 3 1 d' t I d t t'v [Own a Jus mg means mg means yOp m1 6 3,248,664 4/1966 Krasnick et a1. 178-695 X in response to all ofsaid transition pulses and there- 5 after operative in response to onlythose transition pulses which are in time coincidence with one of saidROBERT GRIFFIN Primary Examiner recurring pulses. J. T. STRAT MAN,Assistant Examiner.

10. IN COMBINATION: MEANS RECEIVING A MULTILEVEL CODED DATA SIGNALINCLUDING A TRAIN OF DATA SYMBOLS OCCURRING AT A PREDETERMINED RATE,MEANS GENERATING A TRANSITION PULSE EACH TIME SAID SIGNAL INCLUDES ASIGNAL EXCRUSION BETWEEN ADJACENT ONES OF MULTIPLEINFORMATION-DETERMINANT LEVELS THEREOF, MEANS RECEIVING A TRAIN OFTIMING SIGNALS, MEANS RESPONSIVE TO SAID TIMING SIGNALS GENERATING ATRAIN OF REGULARLY RECURRING PULSES, THE LATTER PULSES OCCURRING AT SAIDPREDETERMINED SYMBOL RATE OF SAID DATA SIGNAL AND HAVING A PULSE WIDTHWHICH IS SUBSTANTIALLY LESS THAN THE PERIOD OF SAID SIGNAL SYMBOLS,MEANS INTEGRATING SAID TRANSITION PULSES, AND